The present invention relates generally to friction hoists and more particularly to a multiple rope friction hoist having at least two wheels mounted on a common axis.
Essentially, a friction hoist is a large diameter wheel around which one or more ropes, for example steel cables, are looped half a turn so that they hang down a hoist shaft, for example a mine hoist shaft, for supporting a pair of conveyances which tend to counterbalance one another. Rotation of the wheel drives the rope or ropes in the manner of a belt on a pulley causing one conveyance to be raised while the other is lowered. The direction of wheel rotation is reversible so that either conveyance can be raised or lowered as required. It is, of course, known to have a system with one conveyance and a counterbalancing weight rather than two conveyances.
The conventional wheel is a large diameter, wide rim, single structure, fixed to a shaft and supported by outboard bearings for rotation on a horizontal axis. The periphery or rim of the wheel has treads which form a groove in which the rope runs and defines a tread wear path. There is, of course, a groove for each rope on the wheel. The tread is usually of a tough, wear resistant material, for example plastic or leather, to provide a good frictional grip on the rope.
The operation of friction hoists, particularly in a mine hoist where the distances for raising and lowering are large, is usually under conditions of considerable acceleration and deceleration which impose severe loading on the ropes. In addition, a mine hoist may have its conveyances loaded and unloaded very quickly, for example by dumping, which may also cause strain on the ropes. In larger installations a plurality of ropes are often used on a friction wheel to provide a safety factor. In this situation, the breaking of one rope will not result in the falling of a conveyance because the remaining rope or ropes are designed to support the load. In some installations it is technically feasible to use a single rope, but safety considerations may dictate otherwise. With a single rope installation, emergency braking means must be provided on the conveyance to engage the wall of the shaft and stop the conveyance if the rope were to break. This type of emergency braking has not been entirely satisfactory in the past and it may not be acceptable where the conveyances are intended to convey people.
As previously mentioned, one alternative to a single rope hoist is to provide plural ropes on the friction wheel to support the conveyances. The ropes are chosen by design so that less than the full number can support the load during emergency conditions if one rope should fail. However, considerable problems are involved in going from a single rope to a plural rope system. For example, in a system having two ropes on the wheel of a friction hoist, the load should be shared equally between the ropes. This is possible, in theory, if both tread wear paths are identically covered and of equal diameters, if both treads wear at exactly the same rate, if wheel deflection affects the treads for both ropes in the same manner, and if both ropes lengthen or shorten in synchronism to the same extent with changes in loading. Obviously, these conditions are not easily met and, consequently, it is difficult to maintain good load sharing between the ropes of a multi-rope winder under all practical hoisting conditions.
Devices for equalizing rope tension are known. These devices are usually mechanical or fluid actuated devices which are difficult to maintain and have inherent limitations. Experience has shown that this is not an entirely satisfactory solution in practice. The ropes tend to behave individually on a wheel that is single unit by winding at slightly different rates. These different winding rates can cause intermittent differences of significant magnitude in the loading of the ropes. As hoists become larger and faster the problems in rope loading increase and may present a limitation on the use of, for example, a friction mine hoist. It is important to ensure that the safe stress level on a rope is not exceeded, and the prior devices cannot always ensure this particularly on large installations.
In a copending U.S. patent application, Ser. No. 754,187, "Unit Hoist," by Peter deHertel Eastcott filed on even date herewith and assigned to the assignee of the present invention, there is described and claimed an arrangement for a friction hoist with two or more ropes in which rope tension is equalized. That invention employs a unit concept where each rope has a separate friction wheel driven by its own motor and a separate braking means. That is, each rope has associated with it a drive motor, wheel and brake to form a unit. The motors are connected electrically to control their driving forces to provide equal incremental rope tensions. The braking means for each wheel in interconnected to provide equal braking. It is essential that both the drive and the braking forces be equalized among the ropes. The arrangement may be used with two or more ropes, and it is particularly suitable for more than two ropes. For an installation where a single rope is adequate, but two ropes are required for reasons of safety, the expense of a separate drive motor for each rope and wheel and of a separate braking system for each rope and wheel, with an equalizing control may not be attractive.